So-called hot melt adhesives are used for various coating and bonding operations such as diaper construction, package forming, automobile parts assembly and various other industrial applications. Generally, it is convenient to store and ship the hot melt adhesive materials in bulk forms such as in the form of chicklets, slats, pellets, bricks or slugs held in containers. In any of these cases, important parameters include the ability to achieve a desired throughput or flow rate of liquid adhesive from the unit and, at the same time, achieve and maintain a relatively precise elevated liquid temperature. This temperature is often referred to as the set point temperature.
Hopper-type melting units may be used to melt and dispense many forms of thermoplastic adhesives including the forms mentioned above. In the case of bulk adhesive in the form of a slug held in a container, hopper-type dispensers or melting units can first remove the slug from the container and then melt the slug of adhesive as it contacts a heated melting grid mounted near the bottom of the hopper. The other forms of adhesive mentioned above may simply be loaded into the hopper. In each case, a heated reservoir is usually disposed beneath the melting grid to receive the fully or partially melted hot melt adhesive as it passes through the grid. Reservoirs have been designed to maintain the adhesive in a heated liquid state suitable for the application and, for this purpose, various types of fins or other heated surfaces have been provided in the reservoir. An outlet of the reservoir typically leads to a pump and heated manifold assembly for pumping the hot melt adhesive to a dispenser appropriate for the application.
Melting grids have been the primary devices for transforming the adhesive from its initial form into a molten or at least semi-molten state. Melting grids may consist of various forms of heated grid members that melt the adhesive on contact. These members are typically elongated fins. Melting grids typically include through passages for the melted adhesive. Preferred grids take the form of aluminum castings with electrical heating elements cast within the grid structure. This maximizes the service life of the heaters and provides the most uniform temperatures at a reasonable cost. Melting grids must be designed to compromise between a number of competing objectives. The primary factors that can be varied to meet these objectives are fin thickness and fin spacing. Fin height is also a factor, but the overall size of the unit and the constraints of casting technology typically limit height.
Thin fins are preferred to maximize the surface area in contact with the adhesive. However, a minimum thickness is required in the vicinity of the heater to allow for variability of the heater location within the casting and variability in the size of the heater itself. A minimum thickness is also required for strength in large units. Thus, the fins are tapered from a relatively thick section near the heater to a relatively thin tip. However, if the taper is too gradual or the fin is too high, the tip will be much cooler than the heater and this will adversely affect the ability to melt adhesive.
Widely spaced fins have been preferred to maximize the cutting force on the solid adhesive and minimize the resistance to flow as the adhesive melts. However, if the fins are spaced too widely, solid adhesive will be able to pass through without melting. Also, widely spacing the fins results in fewer overall fins in a given size grid, and thus less surface area for transferring heat to the adhesive.
Finally, melting units have generally provided a significant space within the reservoir for liquid adhesive to accumulate to meet high instantaneous demand. This is a problem because much of the liquid adhesive ends up spaced a significant distance from the nearest heated surface. The liquid adhesive is therefore difficult to heat to a desired temperature. The fact that hot melt adhesive is a poor heat conductor exacerbates this problem. Its relatively high viscosity precludes significant convective heat transfer as well. Also, in start-up conditions, this adhesive is often in a solid state and must be slowly melted and heated to its set point temperature prior to using the unit.
One way to drive more heat into the adhesive would be to heat the melting elements, such as fins, to a temperature substantially above the desired adhesive temperature. However, the adhesive is likely to char or otherwise degrade under these conditions. This char acts as an insulating layer and degrades the melting performance. It is also likely to break off and clog downstream elements of the system. In addition, degraded adhesive may not provide the bonding performance required by the application. For these reasons, it is desired that the adhesive closely approach the set point temperature of the reservoir, hoses, and applicator heads, and that strong temperature gradients and associated hot spots be avoided. Prior melting units were either relatively small, in which case the residence time in the hoses was sufficient to bring the adhesive up to temperature, or a separate heat exchanger was required between the pump and the hoses. The heat exchanger used the pump pressure to overcome the drag caused by the extensive surface area, but this is relatively expensive as it requires a separate heating zone and an additional high pressure component.
One melting grid which achieves certain advantages in optimizing the above parameters is disclosed in U.S. Pat. No. 5,657,904, which is owned by the assignee of the present invention. The disclosure of U.S. Pat. No. 5,657,904 is hereby fully incorporated herein by reference. This patent discloses a melting grid having intersecting sets of high and low level grid members and certain other unique features which help achieve a balance between melt rate and flow rate. Advances and improvements in this area are nevertheless desirable.
In view of the various problems in this field, it would be desirable to provide a thermoplastic material melting unit in which the melting rate, liquid throughput or flow rate, and overall heat transfer are optimized.